This invention relates to a gas purification process. It is particularly but not exclusively concerned with removing gaseous impurities of higher boiling point (at 1 atmosphere) than carbon dioxide from a gas mixture containing carbon dioxide, and typically other gases having a lower boiling point than carbon dioxide as well as the impurities. The impurities are typically gaseous compounds of sulphur. The removal of the impurities is accompanied by a significant reduction in the proportion of the carbon dioxide in the gas mixtures.
Gas mixtures which contain such impurities and which are rich in carbon dioxide are, for example, produced by the gasification of carbonaceous materials. Such gas mixtures normally require purification prior to their intended use which may, for example, be for the synthesis of ammonia or methanol; for the production of fuel gas; for hydroforming or liquefaction of hydrogen, or for upgrading hydrocarbon mixtures such as natural gas. In general, complete removal of the impurities is desirable, if not mandatory.
There is also generally a need to remove at least some carbon dioxide from the gas mixture. To avoid pollution of the atmosphere the gaseous impurities separated from the gas mixture will normally have to be rendered innoxious, for example, in the case of compounds of sulphur, by conversion to sulphur. In some processes, a high concentration of sulphur-containing impurities facilitates this conversion and thus selective removal of such impurities from the gas mixture is desirable. As a consequence, some carbon dioxide, practically free from the impurities, will become available as a by-product. It is also desirable that the purification process should keep to a minimum losses of valuable gaseous constituents of relatively low boiling point, for example, methane and hydrogen.
Several commercial processes which substantially meet these requirements are known. For the removal of the impurities and carbon dioxide they all use solvents and absorbents which operate by chemical or physical action. In processes employing solvents, a high circulation rate of the solvent is required. Hence the power consumption and losses of other gaseous constituents by dissolution in the liquid are high. In processes employing chemical absorbents, regeneration of the absorbents at elevated temperatures is required, and fuel costs are high even if heat exchangers are employed. In both kinds of process, solvent or absorbent losses by degradation, vaporisation or leaks can be expensive, and the use of such liquids requires expensive provisions to avoid corrosion of the plant. Finally, currently available means for a selective removal of hydrogen sulphide and carbon dioxide make the purification plant complex, and typically several columns are required for the treatment of the gas mixture with the solvents or absorbents.
Rectification processes for purifying carbon dioxide are known. UK patent specification No. 971 362 relates to a process for removing both more volatile and less volatile impurities from the carbon dioxide. The carbon dioxide is liquefied by heat exchange with `Freon` (Registered Trade Mark) and then rectified in a first column to remove the more volatile impurities as a gaseous fraction. The liquid fraction is expanded into a second rectification column from which the less volatile impurities are removed as a liquid fraction. French patent specification No. 2 158 338 relates to a process in which the carbon dioxide is liquefied by heat exchange with a refrigerant, the liquid is rectified in a first rectification column to remove the more volatile impurities and then rectified in a second column operating at a higher pressure than the first to remove the less volatile impurities. U.S. Pat. No. 4,152,129 relates to a process in which only more volatile constituents are removed from the carbon dioxide by rectification.